Reference is made to the following pending applications having the same assignee as this application: Ser. No. 283,456, filed July 15, 1981; now U.S. Pat. No. 4,442,010: Ser. No. 283,470, filed July 15, 1981; now U.S. Pat. No. 4,417,501: Ser. No. 299,325, filed Sept. 4, 1981; now U.S. Pat. No. 4,421,011.
The referenced applications show construction having the function of providing hydraulic power return of steered wheels from a steered position, utilizing rotary type valves in Ser. No. 283,456 and Ser. No. 283,470. The present invention provides a construction for hydraulic power return utilizing a reciprocating spool valve.
The invention relates to a steering gear having hydraulic power boost, for example, as shown in German Pat. No. 2 24 15 235 (U.S. Pat. No. 3,964,371) having spring means for effecting a neutral position of a valve spool wherein the valve spool is axially shifted by rotation of a worm shaft on which it is carried and the worm shaft having a reciprocal ball nut engaging a gear sector for actuating a steering linkage. In order to permit relative axial movement between a steering spindle, which is rotated by manual force on a steering wheel, and the worm shaft, a tongue and slot coupling is utilized. In such constructions, the centering of the valve spool to neutral position is effected by a collar on the worm shaft biased by oppositely disposed compression springs engageable with respective fixed abutments in the housing on respective sides of the worm shaft collar.
Valve constructions as described above are frequently used in heavy vehicles which have several steering adjustments for the steered wheels. These prior steering arrangements as in lighter vehicles are designed to automatically return wheels from a steered position to straight ahead travel position when manual force on the steering wheel is released. The axis geometry of the adjustments of the vehicle steered wheels effects such straightening, for example, an adjustment of king pin slant. However, there is a disadvantage in relying on the axis geometry of the steered wheels for return to straight position, since manual force must be used in steering the wheels to overcome resistance caused by axis geometry, and this is particularly disadvantageous in the case of heavy vehicles having slow straightening return.